Each laser comprises a laser active region called gain in which the supplied energy is converted to coherent radiation by means of stimulated emission. On this behalf an optical resonator is necessary which re-injects part of the radiation back into the gain region. This resonator comprises at least one feedback element, typically a semitransparent mirror. The resonator determines the physical characteristics of the laser radiation by means of its geometry and feedback characteristics. The most prominent radiation characteristics are the spatial profile, the wavelength, and the polarization. The achievable parameters strongly depend on the gain material and the resonator and are usually inversely correlated with each other and the achievable output power. Improving single parameters generally deteriorates others. Generally speaking all losses and not usable emission of light is harmful for the overall performance.
Semiconductor lasers are of outstanding practical importance because they are small and cheap. They directly convert electrical power into light, they have a high conversion efficiency, and they can be produced in large quantities by means of well established techniques of semiconductor technology. In those devices the optical resonator is usually integrated by means of dielectric reflecting coatings on the outcoupling facets or by means of epitaxially integrated refractive index gratings. However, presently the maximum achievable output power or the power density, respectively, is way too low for numerous applications. This is due to the fact that the generation of light takes place in a volume smaller than 1 mm3. On increasing the pump powers the resulting optical power densities would destroy the device. The apparent solution to increase the active volume is practically limited because the modal selectivity of the resonator decreases and the beam quality deteriorates correspondingly. Neither does it help much to introduce substructures into the gain material as disclosed in German publications DE 43 38 606 and DE 36 11 167.
Therefore it has been pursued with semiconductor lasers to separate the resonator off of the gain. In other words the semiconductor gain material is inserted into an external optical resonator. By this a drastic increase in power density could be achieved as disclosed in the following publications: DE 101 61 076, WO 02/21651, WO 02/082593, WO 98/56087, U.S. Pat. No. 4,426,707, Opt. Lett. 27(3) pp. 167-169. All these publications have in common that the emission of the semiconductor is separated into two angular regions out of which one serves as feedback branch and the other one is coupled out to extract the usable light.
External resonators for high-power diode-lasers according to the prior art use a certain direction of emission of radiation which emanates from the gain material exclusively for feedback and a different direction of emission exclusively for extracting usable light. The fraction of radiation that is emitted towards the feedback cannot be influenced and, therefore, is too high or too low as compared to the optimal amount. If the fraction towards the feedback is too high, that is it is above the saturation, the output power lacks this excess power. But if it is too low then the beam quality and side mode suppression suffer. Also the fraction of incoherently emitted light, the so-called amplified spontaneous emission, increases. And in any case the power flux inside the gain is distributed very asymmetrically between feedback and outcoupling branch.
The present invention is directed to providing setups or arrangements which allow for a freely selectable distribution between feedback and outcoupling and an optimizable degree of feedback. It needs to be possible to symmetrize the two channels and adjust feedback.